[DJANGO 3.2] Link in django admin to foreign key object

import operator

from django.contrib import admin, messages
from django.contrib.admin import ModelAdmin
from django.urls import reverse
from django.db import transaction
from django.db.models import Q, QuerySet
from django.forms import TextInput
from django.utils.html import format_html



class FKLinkWidget(TextInput):
    """Widget to show html link for FK field instead of default option field"""

    NO_VALUE_TEXT = 'None'

    def __init__(self, attrs=None):
        self.app_label = None
        self.model_name = None
        self.pk = None
        self.repr = None
        super().__init__(attrs)

    def set_obj(self, obj):
        self.app_label = obj._meta.app_label
        self.model_name = obj._meta.model_name
        self.pk = obj.pk
        self.repr = str(obj)

    def render(self, name, value, attrs=None, renderer=None):
        if self.pk:
            orig_input = super(FKLinkWidget, self).render(name, value, attrs, renderer)
            view_name = f"admin:{self.app_label}_{self.model_name}_change"
            link_url = reverse(view_name, args=[self.pk])
            hidden_input = orig_input.replace('type="text"', 'type="hidden"')
            link_tpl = '<a href="{}" target="_blank">{}</a>'
            return format_html(link_tpl + hidden_input, link_url, self.repr)
        else:
            return format_html(value or self.NO_VALUE_TEXT)


class CustomModelAdmin(admin.ModelAdmin):
    """extendable ModelAdmin which provides several custom attributes

    - fk_links = list of FK fields that should be shown as read-only links on detail page
    this can prevent loading all choice options by django admin, which results 504 http error

    """
    fk_links = []

    def __init__(self, model, admin_site):
        super().__init__(model, admin_site)
        intersect = set(self.fk_links).intersection(self.readonly_fields + self.raw_id_fields)
        if intersect:
            raise ValueError(f'CustomModelAdmin fields: {intersect} are in readonly or raw_id fields')

    def get_form(self, request, obj=None, **kwargs):
        self.obj = obj
        form = super().get_form(request, obj, **kwargs)
        return form

    def formfield_for_dbfield(self, db_field, **kwargs):
        formfield = super().formfield_for_dbfield(db_field, **kwargs)

        if db_field.name in self.fk_links:
            fk_obj = self.get_fk_obj(db_field)
            if fk_obj:
                formfield.widget.widget.set_obj(fk_obj)

            # we disable any actions for that field
            formfield.widget.can_add_related = False
            formfield.widget.can_change_related = False
            formfield.widget.can_delete_related = False
        return formfield

    def get_fk_obj(self, db_field):
        if self.obj:
            fk = getattr(self.obj, db_field.name)
            if fk:
                return fk

    def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
        if db_field.name in self.fk_links:
            kwargs["required"] = False
            kwargs["widget"] = FKLinkWidget

            fk_obj = self.get_fk_obj(db_field)
            queryset = db_field.remote_field.model._default_manager.all()
            if fk_obj:
                kwargs["queryset"] = queryset.filter(pk=fk_obj.pk)
            else:
                kwargs["queryset"] = queryset.none()

        formfield = super().formfield_for_foreignkey(db_field, request, **kwargs)
        return formfield


# usage

@admin.register(UserProfile)
class UserProfileAdmin(CustomModelAdmin):
    fk_links = ['user',]